1. Field of the Invention
The present invention relates to the field of vehicle lighting systems and, more specifically, to a glare control system for reducing headlight glare from oncoming vehicles.
2. Description of the Prior Art
Headlight glare is a persistent safety hazzard in night driving and over the years many systems have been proposed to suppress headlight glare.
In general, the prior art systems fall into two broad categories. In the first, polarizing filters placed over the headlights polarize the emitted light at a 45.degree. axis and a similar polarizing filter is used as a visor through which the operator views the roadway. When a similarly equipped vehicle approaches, the light emitted therefrom is cross-polarized with respect to the visor thereby reducing glare. For such a system to be effective, however, it must be adopted universally. Representative examples of the polarizing system may be found in U.S. Pat. Nos. 1,786,518; 2,230,262; and 2,423,321. Also see commonly-assigned U.S. Pat. No. 2,458,179.
The second type of glare control system may be referred to as the strobe type. In the strobe system, the headlight is rapidly flashed on and off, either electrically by interrupting power or electromechanically by providing an oscillating or rotating shutter in front of the headlights, and the operator views the roadway through a visor that is rapidly switched between light transmissive and opaque states in synchronism with the headlights. The headlights operate above the eye flicker rate and generally are on for a very short portion of the headlight cycle. For example, the headlight may be turned on for 10% of the cycle and be off for 90%. The visor is transmissive while the headlight is on and then is rapidly switched to the opaque state while the headlight is off. Obviously, the light output of the headlight must be 10 times greater than normal to provide sufficient illumination. Because the visor is transmissive for only 10% of the visor cycle, headlight glare from oncoming vehicles is reduced by 90%. Despite the complexity of the strobe system, it has a major advantage in the fact that it need not be universally adopted to provide the benefit of glare suppression.
For representative examples of early strobe type systems, reference may be had to U.S. Pat. Nos. 2,131,888; 2,139,707; and 2,755,700. A more contemporary version of the strobe type glare reduction system may be found in The National Highway Traffic Safety Administration Report PB-257-431 of September 1976 entitled "Advanced Headlighting Systems". The visor used in that system is of the electro-optical type, such as the PLZT shutter described in U.S. Pat. No. 3,245,315.
While both the polarizing and strobe type glare control systems are effective to reduce glare, the inherent light transmission losses of those systems generally tend to deprive the vehicle operator of the benefit of supplemental ambient illumination provided by streetlights especially when there is no vehicle approaching or when the glare intensity is relatively low from oncoming vehicles off in the distance.
The polarizing system described in the previously noted U.S. Pat. No. 2,230,262 addresses the problem by configuring the headlight and visor filters in venetian blind arrays that are switched between closed and opened positions in accordance with the glare intensity of the oncoming path as measured by a photoelectric glare level detector mounted on the front of the automobile. When the glare level is above a predetermined limit, the filters are closed for maximum glare reduction. When the glare intensity falls below the limit, the filters are opened to take advantage of ambient illumination. However, because this is a bistable system (the filters are either opened or closed) which does not adjust proportionally to variations in glare intensity, it would seem that the abrupt changes in perceived roadway illumination may prove tiring to the vehicle operator.
The strobe type systems in the prior art generally do not make any provision for varying the system response in accordance to glare intensity and the transmissive-to-opaque time ratio of the visor cycle is fixed to coincide with the light emitting and nonemitting intervals of the headlight cycle for maximum glare reduction.
Therefore, it is an object of the present invention to provide an improved glare control system of the strobe type that is responsive to the actual glare intensity of the oncoming path and is automatically adjustable to vary the transmissive/opaque ratio of the visor in accordance with glare intensity so that the vehicle operator may benefit from ambient illumination at appropriate times.
It is another object of the invention to provide a method of reducing headlight glare wherein said ratio is adjusted to vary the apparent density of the visor in accordance with the measured glare intensity of the oncoming path.
Other objects of the invention will, in part, be obvious or will, in part, appear hereinafter.